<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .d1{
            width: 100px;
            height: 100px;
            background: #ccc;

            /* border: 2px solid red; */

            outline: 10px solid blue;

        }

        input{
            outline: 0;

            border: 0;

            border-bottom: 1px solid #333;
        }
        
        /* 伪类：获取焦点时 */
        input:focus{
            border-bottom-color: blue;
        }
    </style>
</head>
<body>
    <div class="d1">前端</div>
    <hr>


    用户名：<input type="text">
</body>
</html>